Prakt. Met. Sonderband 46 (2014) 335
ANALYSIS OF STAGE-I-CRACK TIP STRAIN FIELDS BY HIGH
RESOLUTION STRAIN MEASUREMENT IN THE SEM
urch Ball-on-
ie zu Pitting F. Schäfer, P. Grünewald, L. Weiter, M. Thielen, M. Marx, C. Motz
beschrieben,
ı auf. In einer Department Materials Science and Engineering, Saarland University, Saarbruecken,
n Atzkontrast Germany (all authors)
y-Wings sind
Mikrorisse an
dung dürfte ABSTRACT
ım Geflge-
nicht NME's, Crack tip strain field measurements contribute eminently to the evaluation of analytical
crack models and simulations to predict crack growth. Digital Image Correlation (DIC) in
combination with scanning electron microscopy using carefully adjusted acquisition
parameters is a useful tool for the investigation of local strain fields. The usual
assumptions of analytical crack models apply only to stage-l-cracks but most of the
experimental investigations are performed for stage-Il-cracks. In this work DIC was used to
measure the static and cyclic strain field and the plastic zone size in front of a stage |
n RCF Life of fatigue crack on a scale in the lower micrometre range for the first-time to deliver valuable
input for the improvement of models and simulations. This method was compared to both
nce, Vol. 57, established experimental methods like electron channeling contrast imaging (ECCI),
optical microscopy and predictions using the crack model proposed by Bilby, Cottrell and
cyclic rolling Swinden.
5, 1997
Js: The effect
logy, Vol. 30,
1. INTRODUCTION
0 and 18-4-1
Several strategies to measure strain and stress fields of cracks have been developed
Speed Steel”, during the last decades. Techniques like precision-strain-gage-strip measurements at long
+ cracks allow strain field measurements on the millimetre and upper micrometre scale. But
Perspective”, these methods are limited by their lateral resolution. High resolution Microscopic Moire
Interferometry in combination with SEM imaging as a displacement measurement method
ce”, Materials achieves a lateral resolution of 2 to 5 um with a displacement sensitivity of about 200 nm
[1].
ontact fatigue Digital Image Correlation (DIC) was multiply used to increase the spatial resolution of
|. 28, Issue 1, standard techniques for strain measurement during the last years; DIC was used for
residual stress analysis by hole drilling combined with focused ion beam technique for
olling contact instance [2]. The spatial resolution of DIC is only limited to the resolution of the input
‚1099-1110 images. To measure displacements due to elastic strain a high sub-pixel shift
"U Delft, 2010 measurement accuracy is needed and therefore shifts of sub-pixel size must be
cumulation of measurable.
tact Fatigue”, Many authors have investigated the strain fields in front of stage-ll-cracks of several
millimetres length [3] but an investigation for stage-I-cracks with sizes on the scale of the
Evaluation of microstructure is still missing but essential to understand the mechanisms of crack growth.
CP”, Tribology Stage | cracks grow with relatively low stress intensity factor ranges. For these short
fatigue cracks with lengths of less than 300 pm the maximum stress intensity factor is
1 Raumfahrt“, smaller than for long cracks at comparable crack growth rates, resulting in a much smaller
ıftfahrt.pdf strain field and therefore the strain field measurement requires a very high spatial
